1. Field of the invention
This invention relates to a sewing machine including a sewing machine motor for driving a main shaft and a shuttle driving motor for driving a thread loop capturing shuttle independent of the sewing machine.
2. Description of the related art
There have conventionally been provided vertical axis drive type sewing machines comprising a single sewing machine motor for driving both a main shaft for vertically driving sewing needles and thread take-up levers, and a lower shaft for driving thread loop capturing shuttles such as full-turn shuttles. On the other hand, Japanese patent publication Nos. 60-21750-B and 3-234291-A each disclose a hook shaft independent drive type sewing machine comprising an independent shuttle driving motor for driving a thread loop capturing shuttle as well as a sewing machine motor for driving a main shaft. In the independent hook shaft drive type sewing machines, the sewing machine motor and the shuttle driving motor are controlled so that the shuttle is driven in synchronism with the main shaft of the machine. Furthermore, the shuttle is controlled to be driven according to various sewing conditions.
For example, in embroidery sewing machines, a leading end of needle thread is usually open when a sequence of sewing operations is started or when the sewing operation is re-started after a thread of one color has been changed to a thread of another color during the sewing operation. Accordingly, a thread loop captured by a beak of a rotating hook is often loosened immediately after start or re-start of the sewing operation. In such a case, the loosened thread loop is sometimes caught between the rotating hook and an annular rib or race which is formed on an outer periphery of a rotating hook bobbin case holder to prevent fall-off of the holder from the rotating hook. This is referred to as "thread jamming." A generally small-sized motor is used as the shuttle driving motor and has a small torque in the independent shuttle drive type sewing machines. As a result, the shuttle and the needle are desynchronized relative to each other or the full-turn shuttle is abruptly stopped at one rotational position. Accordingly, in order that the needle in a downward movement may be prevented from colliding against the shuttle, the sewing machine motor is stopped upon detection of loss-of-synchronism of the shuttle, so that the sewing is interrupted. Upon stop of the sewing machine motor, an operator opens a protecting cover of the full-turn shuttle to find the thread jamming. Then, the operator manually reverses the full-turn shuttle to eliminate the thread jamming. Thereafter, a reset switch is operated so that the sewing machine is released from an error mode and so that the main shaft and the full-turn shuttle are initialized so as to return to the respective rotational positions for the start of the sewing. The sewing can be re-started upon operation of the sewing start key in the initialized state.
Thus, in the above-described independent hook shaft drive type sewing machines as disclosed in Japanese patent publication Nos. 60-21750-B and 3-234291-A, the elimination of the thread jamming in use of the full-turn shuttle requires the reverse rotation of the shuttle, operation of the reset switch, and operation of the sewing start key so that the sewing is re-started. The operation for the elimination of thread jamming and restart of the sewing is complicated in the independent hook shaft drive type sewing machines. Furthermore, it takes much time until the thread jamming is eliminated and the sewing is re-started. Since the sewing machine is stopped for a long time, the sewing efficiency is reduced.